1. Field of the Invention
The present invention pertains to an electromechanical release mechanism to be used in a circuit interrupting device such as a circuit breaker and in particular in a DC (direct current) circuit interrupting device.
2. Description of the Related Art
DC circuit interrupting devices generally comprise a stationary contact element and a movable contact element. Under normal conditions, these contact elements touch each other and electric current is conducted through them. To interrupt the current, the movable contact element is moved away from the stationary contact element thanks to a release mechanism.
Generally, the release mechanism opens the circuit interrupting device when a defined current through the circuit interrupting device is exceeded. It is usually a passive device to offer the highest level of protection and operates even on loss of auxiliary supply voltage. Most direct release mechanisms are electromechanical and use the magnetic field created by the current in the main circuit to activate a mechanical or magnetic trip system which moves the movable contact element away from the stationary contact element and opens the circuit interrupting device thus breaking the current in the main circuit.
One of the main requirements of the release mechanism is the speed at which it is activated. Because faults on a DC circuit, such as a traction network, can have high initial rate of rise (of about tens of kilo amperes per millisecond) these release mechanisms have to start opening the circuit interrupting device in less than five milliseconds in order to comply with international standards.
The majority of DC circuit interrupting devices, as the one used for traction applications, have fault or overcurrent conditions that are either non existent in the reverse direction of the main current or similar in the reverse direction of the main current and for this reason bi-directional release mechanisms are commonly used in these DC circuit interrupting devices. A bi-directional release mechanism operates in the same way in both directions of the current by using the magnetic flux from the main circuit with the current flowing in either direction to activate a mechanical trip.
There are however several protection standards which call for a unidirectional release mechanism that is actuated only upon detection of a reverse current. This means that the release mechanism will be activated and open the circuit interrupting device when the current flows through the said device in a first direction (reverse direction), but will not be activated by a current flowing in a second direction (forward direction), even under short circuit conditions. There may be a level in the forward direction for which the release mechanism will be activated but this is normally a fairly high value (which may be about 100 kA) in order to protect the circuit interrupting device itself from damages.